We found that the transcription factor FoxO controls neuronal microtubule organization and neuronal morphogenesis in Drosophila. Arguing for evolutionary conservation, FoxO homologs have also recently been implicated in neuronal morphogenesis in worms and mice. In these systems, FoxOs regulate polarization, outgrowth, and morphology-all processes demanding appropriate microtubule organization. Our work demonstrated that Drosophila FoxO limits microtubule stability (or alternatively, promotes microtubule destabilization) at the NMJ. We also found that FoxO protein levels are strikingly reduced in several cytoskeletal stress paradigms, arguing that FoxO dynamically regulates the microtubule network. We propose that FoxO levels are reduced as part of a cellular strategy to counteract microtubule perturbation. This hypothesis is based on FoxO's microtubule- destabilizing function as revealed by phenotypic analysis of foxO mutants. Our identification of FoxO as both a regulator of the neuronal microtubule network, and a neuronal stress-regulated transcription factor, argues that FoxO contributes to the neuronal response to cytoskeletal disruption. In this proposal we will define FoxO signaling systems to gain insight into mechanisms that regulate stability and plasticity of neuronal morphology. Moreover, neuronal microtubule dysfunction is linked to debilitating neuronal pathologies, including motorneuron and neurodegenerative diseases, as well as age and disease-related neuropathies. Thus, analyzing the dynamic regulation of microtubule organization will shed light on mechanisms important to human health and aging. We present a systematic set of experiments to establish FoxO-dependent pathways. We will determine the role of the lipid phosphatase PTEN in promoting FoxO activity at the NMJ, and the role of the E3 ubiquitin ligase Nedd4 in FoxO degradation following cytoskeletal perturbation. Furthermore, we present phenotypic analysis and expression data arguing that the microtubule - associated protein CRMP (Collapsin Response Mediator Protein) is repressed by FoxO. Thus, we will establish the role of CRMP in regulating microtubule stability in motorneurons, and define its relationship with FoxO. Finally, we will establish the FoxO's function in dendritogenesis and in aging neurons, to define the scope of its neuronal phenotypes, and to illuminate FoxO function in these diverse neuronal contexts.